1. Field of the Invention
The present invention relates to a method for surface treatment of an electrical steel sheet for forming an electrically insulating film, which electrical steel sheet exhibits improved blanking property, lamination weldability, and electrical insulative characteristics.
2. Description of the Related Arts
To decrease the eddy current loss of electrical steel sheets, the sheet thickness is reduced and the sheets are provided with an insulating film on the surface thereof. In the manufacture of cores for motors for domestic electrical goods and transformers, the electrical steel sheets are continuously formed into blanks having the shape of a core, a predetermined number of the blanked sections are laminated together, and the edges of the resultant laminate core are fixed by welding. The insulating film is, therefore, required to exhibit not only an insulative property but also a member of other necessary properties, such as blanking capability, adhesion, lamination welding, heat resistance, and resistance against oil. It must also provide a high space factor. A number of patents disclose methods of forming the insulating film which allegedly satisfy these properties.
A known method, in which phosphate or chromate is used as components for forming an insulating coating, provides good heat-resistance and weldability but the blanking and adhesion properties are not satisfactory.
In another known method, which aims to improve the blanking property, organic matter, such as water-soluble resin or emulsion resin, is added to the inorganic treating agent such as phosphate and chromate. The added organic matter greatly improves the blanking property, but when the laminated sections are welded, the organic matter decomposes to yield gases. As a result, blow holes are formed in the weld bead. The weldability of the film formed by this known method is therefore very unsatisfactory.
Japanese Examined Patent Publication No. 49-19078 aims to satisfy both the blanking property and the weldability requirements and proposes to form on an electrical steel sheet an insulating film having a surface roughness H.sub.max of 2.mu. or more. The H.sub.max of 2.mu. more is expressed by an Ra (the center line average roughness stipulated in JIS B0601) of approximately 0.5 .mu.m or more. The relationship between the surface roughness of an insulating film and the lamination weldability is explained hereinbelow with reference to FIG. 1.